In the past, a method of mapping based on shade detection (i.e., a shade sensor scanning method) has been known as the method of detecting the conditions of the plurality of wafers stored in the wafer cassette (or FOUP). Namely, this method is intended for mechanically scanning each edge portion of the wafers by using a transmission light switch.
In addition, another method of detecting the conditions of the wafers by taking an image of the wafers stored in the wafer cassette by using a camera and then processing the image has been known. As one technique utilizing the image acquired by such a mapping sensor, there is a method of obtaining and analyzing multiple brightness profile lines, each observed between two points perpendicular to each wafer surface.